A battery is required to power an electronic device when there is no constant source of electrical power or when it is intolerable for the source of electrical power to be interrupted. Whenever a battery-operated device is used, it is desirable to maximize its operating life. One way of doing this is to connect a number of batteries in parallel. One problem with connecting batteries directly in parallel is that current will flow from the battery with the highest voltage to the battery with the lowest voltage. Such a current may cause one or more of the batteries to burst into flame, explode, or both. A diode may be used to prevent current flow from one battery to another. Unfortunately, a diode will also cause an undesirable voltage drop. Such a voltage drop may require the use of a higher voltage battery. To increase the voltage of a battery typically requires an increase in the number of voltaic cells that make up the battery. Therefore, increasing battery voltage typically results in an increase in the size and weight of the battery, both of which may be at a premium in a battery-operated device.
Another problem with batteries is determining how many hours of operation are left on a battery. Battery voltage alone is not a good indicator of how much operating time is left on the battery. Typically, the current drain per hour on the battery is measured, the total current capacity of the battery is estimated, and the second number is divided by the first number to arrive at the number of hours of operation for the battery. Such a calculation is difficult and inaccurate if the current drain on the battery is not constant.
U.S. Pat. Nos. 3,666,961, entitled "ELECTRICAL POWER SUPPLY"; 3,666,962, entitled "ELECTRICAL POWER SUPPLY"; 5,764,032, entitled "MULTIPLE BATTERY SWITCHOVER CIRCUITS"; and 5,898,291, entitled "BATTERY CELL BYPASS TOPOLOGY," each disclose a device for switching between batteries. However, U.S. Pat. Nos. 3,666,961 and 3,666,962 use high current devices such a solenoid and a current meters which could not maximize the operating life of a battery-operated device as does the present invention. U.S. Pat. Nos. 5,764,032 and 5,898,291 do not provide the functionality as does the present invention. U.S. Pat. Nos. 3,666,961; 3,666,962; 5,764,032, and 5,898,291 are hereby incorporated by reference into the specification of the present invention.
U.S. Pat. Nos. 4,622,508, entitled "LITHIUM BATTERY PROTECTION CIRCUIT"; 5,610,495, entitled "CIRCUIT AND METHOD OF MONITORING BATTERY CELLS"; and 5,894,212, entitled "DISCHARGE MONITORING AND ISOLATING SYSTEM FOR BATTERIES," each disclose a device for testing and switching out series-connected individual voltaic cells that make up one battery and does function on multiple batteries connected in parallel as does the present invention. U.S. Pat. Nos. 4,622,508 and 5,610,495 are hereby incorporated by reference into the specification of the present invention.